This invention relates to tomography of the type described in GB-2119520B in which tomographic images of a body are constructed by placing a plurality of surface electrodes at spaced intervals on the body, causing currents to flow in the body, and measuring the potentials between pairs of electrodes, calculating the potential in each case on the assumption that the body consists of one uniform medium, plotting the isopotentials corresponding to the calculated results to create a uniform image of the body, obtaining the ratio between the measured potential and the calculated potential in each case, and modifying the image in accordance with the respective ratios by increasing the impedance along an isopotential in proportion to a ratio greater than unity or decreasing the impedance in proportion to a ratio less than unity.
The calculations of potentials and the obtaining of ratios have been carried out using a computer, and the plotting of the isopotentials have been carried out by a visual display unit (VDU) and/or a print-out run off the computer.
An additional method is provided for indicating a change of state in the body in GB-2160323B, and this method includes the step of determining ratios between initial and subsequent measured actual potentials between electrodes.
The voltage measured between a pair of electrodes connected to a body surface when a current is applied between a second pair of electrodes applied to the body surface depends on (a) the relative positions of the electrodes on the surface; (b) the shape of the body surface and (c) the internal impedance distribution within the body. (a) is the dominant factor followed by (b) and then (c). However the use of a calculated or measured reference set allows the contributions from (a) and (b) to be largely eliminated from the image reconstruction calculation; the differences in the sets of measurements between the `reference` set and the `data` set are dependent almost solely on the changes in internal impedance.
Both methods are described in U.S. Pat. No. 4,617,939 and both methods assume that the cross-sectional shape of a body is circular and thus produce circular images, which may not be satisfactory if the true cross-sectional shape is far from circular, as is frequently the case.